Field of the Invention
The disclosure relates to the field of power electronics, and more particularly, to a switching mode converter having multiple output terminals.
Background of the Invention
In conventional techniques, a driving power source for an LED back light typically has multiple output terminals. At least one constant current output port is used for providing a driving current to an LED load, and at least one other constant voltage output port is used for supplying electric energy to the system and other loads.
FIG. 1 is a schematic circuit diagram of an example switching mode converter having multiple output terminals according to the prior art, including a constant voltage output port and a constant current output port. As shown in FIG. 1, the switching mode converter 1 having multiple output terminals provides multiple outputs by a plurality of secondary-side windings of a transformer. Each secondary-side winding is connected to an output circuit. In a constant current output circuit, the secondary-side winding Ns1 is coupled to a primary-side winding Np. A primary-side power switch Q1 is connected in series with the primary-side winding and is controlled to be turned on and off, so that a square wave voltage is generated between two ends of the secondary-side winding Ns1. A diode D1 and a capacitor C1 constitute a rectifier circuit which rectifiers and filters the square wave voltage and provides a relatively constant output voltage Vo1. An inductor L, a power switch Q2, a diode D3 and a capacitor C3 constitute a boost-type switching mode converter, which is connected between the rectifier circuit and an LED load. The boost-type switching mode converter is controlled by a constant current control circuit 11 to convert the voltage Vo1 into a constant output current ILED and supplies it to the LED load. The constant current control circuit 13 generates a control signal in accordance with a current sampling signal Vs1 and a current reference signal Iref. Meanwhile, in the constant voltage output circuit, a secondary-side winding Ns2 is coupled to the primary-side winding Np. A diode D3 and a capacitor C3 constitute a rectifier circuit which rectifies and filters a voltage and provides a constant output voltage Vo2. A voltage sampling signal Vs2 representing the output voltage Vo2 is transferred to the primary side by an optocoupler circuit 11. A constant voltage control circuit 12 controls on and off states of the power switch Q1 in accordance with the voltage sampling signal Vs2 and a reference voltage Vref so that the output voltage Vo2 maintains constant. Thus, the switching mode converter as shown in FIG. 1 provides a constant current output and a constant voltage output by two-stage power stage circuit.
However, the two-stage power conversion in the constant current output circuit increases conversion loss and decreases conversion efficiency. Moreover, the two-stage power circuit needs two magnetic components, which increase a size of the system and are disadvantageous for miniaturization.